The present invention relates to a ballast circuit for a fluorescent lamp. More precisely, the present invention relates to a compact ballast circuit having a capacitor in series with an inductor, such that the fluorescent lamp is interposed in series with capacitor and the inductor, and particularly where the inductor is formed using materials which are commonly available for use in transformers but having only a single winding.
Fluorescent lamps are becoming increasingly popular for use in the home or office because of their high operating efficiency as compared to incandescent lamps. Indeed, fluorescent lamps emit light at several times the efficiency of a typical incandescent lamp, and do not generate as much heat as a typical incandescent bulb, thereby conserving radiant energy and eliminating excess heat output.
A typical fluorescent lamp is constructed from a glass tube which contains two electrodes at opposite ends, a coating of powdered phosphor covering the interior of the tube, and small amounts of mercury. The major portions of a fluorescent lamp are the bulb, electrodes, fill gas, phosphor coating and a base used to support the external conductors of the electrodes. When energized, the electrodes provide a large potential between which free electrons initiate an arc. The arc generates some visible radiation, but mostly ultraviolet radiation, which in turn excites the phosphor coating causing it to emit light. In this process, the fluorescent effect is caused by the mercury when it is vaporized in the arc.
These fluorescent lamps require a ballast. A ballast is necessary to maintain constant current flow into the lamp. In a ballast resistor, for example, the resistance increases as the temperature increases. As resistance increases, less current is allowed through, thus lowering the temperature and consequently lowering the resistance. Current flow through the lamp is thus maintained at a constant level.
Larger fluorescent systems are well known and have involved larger scale, well known illumination electronics which have been optimized to one degree or another over the years. The techniques for optimizing these larger fluorescent systems include a variety of issues, especially pertinent to eliminating flicker, providing light which is suitable for reading and working, and providing adequate lighting in large industrial facilities. With smaller fluorescent lamps of lesser luminous intensity, in which less visible lighting is required, the circuit details must enable operation with efficiencies as high as the larger fluorescent systems.
As fluorescent lighting has gained acceptance for increasingly less formal and consequently smaller uses, the sizes of the fluorescent lamps and their systems have become smaller. The reduced scale of the fluorescent lights cannot generally be accomplished with a simple size reduction in the circuitry elements. Care must be taken to re-design the operating electronics to insure first that a size reduction may be had, and second that the power efficiency is present in the smaller sized lamps. This is especially so, since it is the power efficiency which generated the interest in a more widespread use of fluorescent lighting initially.
One such technique involved in down scaling is described in U.S. Pat. No. 5,179,323 to Byung L. Ham, entitled "Ballast for Mini Fluorescent Lamp," which issued on Jan. 12, 1993. The technique described therein uses a transformer having a shorted secondary winding to attempt to mitigate the effects of inductive heating. However, with regard to the cost of materials involved in mass production of mini-fluorescent lamps, a small savings per lamp translates into significant savings in view of the larger numbers of mini lamps produced. The use of a transformer secondary in a closed loop fashion is associated with a significant cost including the cost of the wire for the secondary winding, the cost of its coating, and the cost of winding it on a transformer core without damaging the primary winding, and the labor and extra process time involved. Further, the use of a transformer with a shorted secondary requires a secondary impedance match as the energy is transformed through the transformer.
The use of a ballast circuit containing a shorted secondary would require a change in the transformer if the operating conditions changed, such as a change to a higher supply voltage. With a higher supply voltage, the secondary current would significantly increase causing an even more significant increase in the waste power in the secondary according to the relationship P=I.sup.2 R. Thus, a new, two winding transformer would be needed for each size mini-fluorescent lamp and for each level of input power.
What is needed is a very simple mini-fluorescent ballast circuit, one has an inductive element which eliminates the significant cost associated with a secondary winding, and preferably which will enable the use of a single inductive element with a given mini-fluorescent lamp with changing conditions, such as input power. These and other objects are met with the ballast circuit of the present invention.